Sinclair BASIC was originally developed in 1979 to fit in the 4 KB of ROM available on the Sinclair ZX80.

It was initially an incomplete implementation of the 1978 ANSI minimal BASIC standard, and evolved through the 8 KB ROM ZX81 and TS1000 to be an almost complete version in the 16 KB ROM ZX Spectrum. It is present in all ZX Spectrum compatibles.

Currently, interpreters exist for modern operating systems (as well as for vintage systems) that allow Sinclair Basic to be easily used.

They are entered via Sinclair's somewhat unorthodox keyword entry system. The most common commands require just a single keystroke; for example, pressing P causes the entire command PRINT to appear. Less frequent commands require more complex key sequences: BEEP (for example) is keyed by pressing CAPS SHIFT plus SYMBOL SHIFT to access extended mode (later models include an EXTENDED MODE key), keeping SYMBOL SHIFT held down and pressing Z. Keywords are colour-coded on the keyboard to indicate which ⇧ Shift-sequence is required.

Commands found exclusively on the ZX81 and its clones, the TS1000 and TS1500 are FAST, SCROLL, SLOW, UNPLOT, GOSUB and GOTO (vs the Spectrum's functionally identical GO SUB, GO TO).

On the ZX Spectrum each reserved word was assigned a character code between 165 and 255 in the latter half of the system character set, and expanded by referencing a token table held in ROM. As a result, any reserved word in a program listing occupied just one byte of memory, a significant saving over traditional letter-by-letter storage. This also meant that the BASIC interpreter could quickly determine any command or function by evaluating a single byte.

The 128k Spectrum models—the ZX Spectrum 128, +2, +3, +2A, and +2B—introduced a conventional letter-by-letter BASIC input system, and two new commands, neither of which was present in or recognised by the machine's legacy version of 48k BASIC:

SPECTRUM, which switched the 128k Spectrum into a 48k Spectrum compatibility mode

The original Spanish ZX Spectrum 128 included four additional commands in Spanish,[2] one of which was undocumented. These can be translated as:

EDIT (to edit a line number or invoke the full screen string editor)

RENUM (to renumber the program lines)

DELETE (to delete program lines)

WIDTH (to set the column width of the RS232 device, but undocumented as the code was broken)

Unlike the LEFT$(), MID$() and RIGHT$() functions used in the ubiquitous Microsoft BASIC dialects for home computers, parts of strings in Sinclair BASIC are accessed by numeric range. For example, A$ (5 TO 10) will give a substring starting with the 5th and ending with the 10th character of A$. As with modern programming languages such as Python, it was therefore possible to replace the LEFT$() and RIGHT$() commands simply by omitting the left or right array position respectively; for instance A$ ( TO 5) is equivalent to LEFT$(A$,5).

This extension added commands and do a complete memory remap to avoid the system to overwrite the extended screen memory area. Two versions existed: a version for TC2048 and a version for TS/TC2068 because they have different memory map.

^ abcdefINK, PAPER, FLASH, BRIGHT, OVER and INVERSE set attributes for outputting text and graphics to the screen. They can be used either as commands, to apply to all subsequent output until set again, or within a PRINT statement, to apply only from that point until the end of the statement.

^ abcdefCAT, ERASE, FORMAT and MOVE were originally designed to be used with peripherals, but at the launch of ZX Spectrum, they had not been completely implemented, such that their use generated an error message (Invalid Stream). Later with the aid of the ZX Interface 1 shadow ROM, they were used for the ZX Microdrive. (The shadow ROM was paged when the BASIC interpreter detected a syntax error, which is why most ZX Microdrive commands use a "*").

^The control variable of a FOR loop must consist of only one alphabetical character.

^ abUnlike most other BASIC dialects, Sinclair Basic did not include the ELSE operator in the IF–THEN(–ELSE) clause. Thus, instead of:

10IF V=5THENGOTO50ELSEGOTO100

it is:

10IF V=5THENGOTO5020GOTO100

^LET is compulsory (i.e., LET A=1 but never A=1). This practice is also different from most other BASIC dialects.

^ abcdefgString variable names must consist of only one alphabetical character. Thus, LET A=5, LET Apples=5, and LET A$="Hello" are all good, while LET APPLES$="Fruit" is not.

^Machine code could be executed using the USR function, the value provided being the start address of the machine code to execute and the return value being the contents of the BC register pair (unlike most other Z80 based computers that returned the value of the HL register pair). thus:

LET A=USR 30000

would jump to and begin executing the machine code starting at memory address 30000, and on successful completion would store the value of the BC register pair into the variable A which can then be used by the programmer.

^The VAL function does not just evaluate numbers, but also evaluates full expressions. For example, PRINT VAL(A$) will output 14 when given an A$ of "3*3+4+COS(0)". VAL$ does the same but returns a string.